1. Technical Field
The present invention relates to a method of forming a ferrite film on a structural member composing a plant and, more particularly, to a method of forming a ferrite film on a structural member composing a plant which is suitable to application to a boiling water nuclear power generation plant.
2. Background Art
As a power generation plant, for example, a boiling water nuclear power generation plant (hereinafter, referred to as a BWR plant) and a pressurized water nuclear power generation plant (hereinafter, referred to as a PWR plant) are known. For example, the BWR plant has a nuclear reactor with a core in a reactor pressure vessel (hereinafter referred to as an RPV). Cooling water supplied to the core by a recirculation pump (or an internal pump) is heated by heat generated due to nuclear fission of a nuclear fuel material in fuel assemblies loaded in the core. A part of the heated cooling water becomes steam. This steam is introduced to a turbine from the nuclear reactor and turns the turbine. The steam discharged from the turbine is condensed in a condenser, producing water. This water is supplied to the nuclear reactor as feed water. To suppress generation of radioactive corrosion products in the nuclear reactor, a demineralizer disposed in a feed water pipe mainly removes metal impurities from the feed water.
In a power generation plant such as the BWR plant or PWR plant, a main structural member, with which the cooling water is contacted, such as the reactor pressure vessel, are made of stainless steel or nickel-base alloy to suppress corrosion. Further, for the other structural members such as a reactor water clean-up system, a residual heat removal system, a reactor core isolation cooling system, a core spray system, a feed water system, and a condensate water system, carbon steel members are mainly used in view of reducing the necessary cost of building the plant and of avoiding the stress corrosion cracking of stainless steel caused by high-temperature water flowing in the feed water system and the condensate water system.
Furthermore, since corrosion product, which is origin of a radioactive corrosion product may be generated also from the wetted surface of the RPV and recirculation pipe, for the structural members of the main primary-system, anticorrosive steel, such as less-corrosive stainless steel or nickel-base alloy is used. Further, the RPV made of low-alloy steel has a weld overlay of stainless steel on the inner surface to prevent the low-alloy steel from being come in direct contact with reactor water (cooling water existing in the RPV). The reactor water is cooling water existing in the reactor. Furthermore, a part of the reactor water is purified by a demineralizer in a reactor water clean-up system and metallic impurities slightly existing in the reactor water are removed positively.
In spite of countermeasures against corrosion as described above, it is unavoidable that an extremely small amount of impurities is present in the reactor water. Accordingly, some metal impurities are deposited on the surfaces of fuel rods included in the fuel assemblies as metal oxide. The metallic purities (for example, a metallic element) deposited on the surfaces of the fuel rod cause a nuclear reaction due to irradiation of neutrons generated by nuclear fission of the nuclear fuel material in the fuel rods and become radionuclides such as cobalt 60, cobalt chromium 51, and manganese 54. Most of these radionuclides are kept deposited on the fuel rod surfaces in a form of an oxide. However, some radionuclides are dissolved as ions into the reactor water according to the solubility of the included oxide, and other radionuclides are released again into the reactor water as insoluble solid called crud. The radioactive materials included in the reactor water are removed by the reactor water clean-up system. Radioactive materials that have not been removed circulate in, for example, a recirculation system together with the reactor water. Purina this recirculation, the radioactive materials are accumulated on the surfaces of the structural members composing the BWR plant, where the surfaces are come in contact with the reactor water. As a result, radiation is emitted from the structural member surfaces, causing workers in charge of periodic inspection to be exposed to the radiation. Exposure dose is managed for each worker so that it does not exceed a predetermined value. Recently, predetermined values for the exposure dose have been lowered, causing a need to lower the exposure dose for the each worker as much as possible, in an economical manner.
Therefore, many methods of reducing deposition of radionuclides onto inner surface of pipes (structure members) and many methods of reducing the concentrations of radionuclides in the reactor water are considered. For example, a method of suppressing inclusion of radionuclides such as cobalt 60 and cobalt 58 into the oxide film is proposed (refer to Japanese Patent Laid-open No. 58 (1983)-79196). In this proposed method, for example, metal ions such as zinc ions are injected into the reactor water to closely form an oxide film including zinc on an inner surface of the recirculation pipe, with which the reactor water is come into contact.
Further, Japanese Patent Laid-open No. 2006-3843 (US2006/0067455A) and Japanese Patent Laid-open No. 2007-192745 propose a method of suppressing deposit of radionuclide on surface of a structural member after operation of a nuclear power generation plant by, after chemical decontamination, forming a magnetic film as a ferrite film on the surface of the structural member composing the nuclear power generation plant. In this method, a treatment solution that includes a formic solution including iron (II) ions, hydrogen peroxide, and hydrazine is heated in a range from an ordinary temperature to 100° C., and the heated treatment solution is come into contact with the surface of the structure members whereby a ferrite film is formed on the surface. Furthermore, another method is proposed. In this method, a nickel ferrite film or a zinc ferrite film more stable than the magnetic film is formed on the surface of the structural member composing the nuclear power generation plant and after operation of the plant and thus, deposit, of radionuclides on the surface of the structural member is further suppressed.
To suppress the stress corrosion cracking of the structural member composing the nuclear power generation plant, formation of a composite oxide layer of zinc and chromium in which zinc chromite (ZnCr2O4) and chromium oxide (Cr2O3) coexist on the surface of the structural member with which cooling water is come in contact is described in Japanese Patent Laid-open No. 2001-91688.
Japanese Patent Laid-open No. 2000-352597 describes stabilization of Cr included in the structural member composing the nuclear power generation plant. This stabilization of Cr relieves the sensitivity to stress corrosion cracking of the structural member and intends to suppress the corrosion of the structural member. The corrosion suppression can reduce exposure dose during the periodic inspection. Furthermore, Japanese Patent Laid-open No. 2000-352597 proposes that the stabilization of Cr forms a compound in a form of MCr2O4 (N means one kind of or a mixture of several kinds of Zn, Ni, Fe, and Co) on the surface of the structural member. As a forming method of MCr2O4, for example, FeCr2O4 on the surface of the structural member, any one kind or several kinds of the surface processes of plating, coating, lining, flame spraying, prefilming, and grinding are used.